Technologies for measuring surface deformations, or changes in topology in arbitrarily shaped objects include holography, electronic shearography and electronic speckle detection.
These technologies are different from traditional interferometry. In a traditional interferometer, the measurement system is designed to emit a uniphase (typically spatially coherent) beam of light and receive a specular return of the light from an object. The object must have at least one optically smooth surface. The shape of the surface is such that it matches (or nearly matches) the shape of the wavefront of the impinging beam of light over at least a portion of one of the object surfaces.
In contrast, a holographic or speckle test (HOST) is performed with diffusely reflected light, such that the surfaces of the object have no predetermined shape restriction with respect to the impinging beam (or beams) of light.
Optical Shop Testing, Third Edition, Section 16.2, edited by Daniel Malacara, describes holographic and speckle testing of an object that is subjected to stress. As described, when a laser beam is scattered from a diffuse surface, the scattered light has a grainy appearance. The grainy appearance is an interference phenomenon know as speckle. The statistics of a speckle distribution depend upon the statistics of the object surface. If imaged by a lens, the speckle is said to be subjective, and the smallest speckle in the image has a size equal to the Airy disk, 2.44λ/F/# generated by the optical system, where F/# is the working F-number of the system and λ is the wavelength of light. The intensity distribution and the statistics of the speckle pattern are an indication of the roughness of the surface used to generate the speckle pattern. A speckle pattern generated by an object surface may be thought of as the object's fingerprint.
When the object is perturbed in some way, the speckle pattern changes in a predictable way. Two different techniques, which may be used to determine the perturbation of the object, are speckle photography and speckle interferometry. Both techniques involve a comparison of two or more speckle patterns. Both techniques are briefly described below.
Speckle interferometry includes a reference beam to enable measurement of the phase change in the speckles. It is assumed that the speckles from one speckle pattern to another are correlated, so that they do not shift by more than the diameter of a speckle between exposures. Speckle photography looks at the correlation between two speckle patterns where the fringes arise from a translation between exposures. The speckles from a small area of the two speckle patterns (translated with respect to one another) generate fringes in the Fourier plane.
When TV cameras began being used and digital images stored and processed in hardware via electronics, these techniques were first named electronic speckle pattern interferometry (ESPI) and often called TV holography. Because of the low spatial frequency response of TV cameras these techniques measured in-line holograms. As cameras with higher spatial resolutions became available, it became possible to make off-axis holograms. These techniques are known as digital holography or phase shifting ESPI. Digital holography refers to any type of hologram stored and reconstructed digitally, and the term speckle interferometry also refers to the same systems where speckles are present. The mechanisms for creating interference are similar in both, and results are as well. What differentiates one technique from another is how well different spatial frequency structures are reconstructed, and how immune to noise the techniques are.
ESPI uses an optical setup including a TV camera, CCD camera, or detector array placed at the image of the test surface. Because standard video signals are generated, the results of a test may be stored electronically for later viewing and processing. A speckle interferogram of the object is recorded by the camera and stored electronically. A single interferogram may be written asI=I0(1+∂ cos φ)Where I0 is the dc density, ∂ is the visibility, and φ is the phase of the interference between the reference beam and the speckle pattern scattered by the object. After the stress applied to the object is changed, the above becomesI=I0(1+∂ cos φ′)where φ′=φ+Δφ, and Δφ is the phase change. The stored interferogram is then subtracted from exposures recorded after a change in applied stress to the object, and the difference is squared to yieldI2=I02∂2(sin2φ)[sin2(Δφ/2)]
This equation shows that there are fringes due to object displacement Δφ as well as fringes due to the phase of speckles φ resulting from the interference between the reference and speckled object beams.
Because processing for static and dynamic measurements is performed in electronics, fringe data may be obtained with ESPI at video frame rates (25 or 30 frames per second) or faster, depending upon the camera and data acquisition hardware. This speed enables measurements to be made even when the object is not very stable.
Because of the diffuse nature of the reflected light from the object under test, the illumination efficiency of a HOST system is very low. The efficiency is very low due to the fact that the diffusely reflected light scatters substantially from the object (or work piece), typically filling a large solid angle, but the receiving optics (the optics that form the image of the object under test) operates with a much smaller solid angle. The angular efficiency loss due to the solid angle mismatch may be greater than 1000 times (solid angle of diffuse reflection power divided by solid angle of receiving optics).
In addition, because of the angular efficiency loss, conventional HOST systems require very high power illumination lasers (greater that 100 milliwatts of optical output), or the HOST systems are restricted to work over short distances (roughly, over a distance of a meter, or less).
As will be explained, the present invention provides a system and method for measuring surface deformations by including retro-reflective surface treatments on the surface of an object under test. The present invention improves on conventional systems and methods by not requiring a HOST system with a high power illumination laser, nor requiring a HOST system which is restricted to operate over short distances.